A large group of Conservative MPs, among them two members of the cabinet, met recently for a dinner to mark the 20th anniversary of the Tory rebellion against the Maastricht Treaty. By all accounts, they were in a heady mood, intoxicated not so much by the Spanish wine available as their belief that tomorrow belongs to them. Speeches calling for Britain to withdraw from the European Union were cheered with the lusty conviction that their dream of a divorce will turn into a reality.

They didn't acquire the labels Eurosceptic and Europhobe until the bitter battles that began to convulse the Conservative party from the 1980s, but there have always been people who were against Britain having anything to do with it since the idea of "Europe" was invented. Their leading advocates have changed over the years. So have some of their arguments. For periods, they have been most dominant in the Labour party rather than the Tories, but they have always been a vigorous strain in the British political bloodstream. They have often proclaimed that they, not whichever prime minister they were denouncing at the time as a treacherous sell-out, represented the authentic voices of the people of Britain. But the most important point about the outists is that they have always lost. Whenever big European questions have been put to a decisive test, they have been defeated.

It is sometimes forgotten, not least by themselves, that the Maastricht rebels lost. They helped to destroy John Major's government, but the treaty passed through Parliament in the end. Opinion polls have often convinced the outists that they had the British public with them. Yet whenever their proposition has been put to the British electorate, they have been roundly defeated. The Harold Wilson referendum on membership in 1975 resulted in a crushing victory for the yes side. Twice, at elections in 1983 and 1987, the British have been offered withdrawal from the European Union by a major political party. In both those cases it was by the Labour party. On both occasions, Labour went down to a landslide defeat. The most Eurosceptical campaign ever fought by the Tories was under William Hague in 2001. He too went down to a landslide defeat, the scars of which are still borne by the foreign secretary and shape his view that his party should be very careful now.

You can see why the outists think that this time is different. They appear to be close to capturing much of the Conservative party to the point where some cabinet ministers can let it be known that they favour exit and can do so without being sacked, or even rebuked, by the prime minister. The Euroscepticism of the British public appears to have turned into outright phobia. Our poll last weekend had a majority of respondents saying they would probably or definitely vote to leave the EU. Some of our most senior pro-European politicians are increasingly fearful that Britain is sliding towards the departure lounge. Yet for all that, I am beginning to wonder whether the reverse might actually be the case. The outists could be at the peak, or close to it, of their power.

I say this partly because they have had things almost entirely their way for the past two to three years. They have been able to claim vindication for many of their prophecies about the euro. The serial crises that have afflicted the EU have inevitably undermined the whole idea of Europe. The deadlock over the budget at last week's scrappy Brussels summit is just the latest example of that. By contrast, the pro-Europeans in Britain have been cowed, rarely daring to venture the case for continued membership. But this also means that the British have yet to be properly confronted with what it would mean to depart from what remains, for all its many problems, the world's largest economic and political alliance. When they are asked to contemplate the consequences, I strongly suspect opinion will dramatically shift.

As we report today, Tony Blair will make an important intervention this Wednesday. He will tell a business audience that the case for Europe is no longer principally about maintaining peace on this continent but about projecting power in the world. Out of the EU, Britain will be denied representation at the top table when Europe negotiates – on trade, for example – with America and China.

When – or if – the fundamental question is put, it will matter a great deal that the commanding heights of British politics are still occupied by people who believe in British membership of the European Union. Nick Clegg has told colleagues that he will "fight to his last breath" to stop us falling out of the EU. The amount of oxygen in the Lib Dem leader could not alone, of course, prevent an exit. Far from it. A more important component is the position of the Labour party. Ed Miliband has waited more than two years to make a speech about Europe but when he finally defined his position, in an address to the CBI last week, it was worth listening to. The speech leant heavily against a referendum. He has thought about what holding a referendum would mean for a government led by him and concluded that it would be an absolute nightmare. At best, it would be a massive distraction at a time when he would face a huge number of other challenges. At worst, if the country defied his advice to stay in the EU, he would very likely have to resign as prime minister. Says one colleague: "Ed is not going to wreck a Labour government before it has even been elected by making stupid promises now."

You cannot absolutely rule out Labour feeling compelled to make a referendum pledge in the event that the pressure to match a Tory promise of a plebiscite became really intense, but it is now clear that this would be the last choice of the Labour leader. The speech was unequivocal about where he stands, referendum or not. He had serious criticisms to make of the EU. And the temptation for Labour to exploit Conservative divisions to torment David Cameron by making opportunistic alliances with Tory rebels, as it did over the European budget, will sometimes be too much to resist. But on the existential question, the Labour leader could not have expressed his view with more crystal clarity. He called it "a betrayal of the national interest" for Britain to divorce itself from its continent and listed all the reasons, economic and strategic, why.

It was no coincidence that he chose to make the pro-European case to the CBI. The great majority of business leaders, whether they are running British companies or are foreign investors here, think exit would be a disaster. Few of them have yet articulated their alarm in public. One member of the cabinet says: "They are scared of speaking out when the issue is so politically toxic." But in any referendum, the business view that quitting would hazard many jobs, investments and markets would surely be telling with the public. The more sober outists privately acknowledge that the almost complete absence of significant business support is one of their greatest weaknesses.

This is beginning to concentrate some minds in the Conservative party. As was the case in the 1990s, the European issue has the potential to allow Labour to build relations with business while cutting off the Tories from people who would normally be their natural allies.

Sometime before Christmas, or so I am told by his aides, the prime minister will finally make his long-awaited speech setting out his position. There is a general expectation that he will float a referendum on British membership on renegotiated terms, though some of his team say there is no certainty yet about when such a referendum might be held and it is not even absolutely definite that the speech will make a cast-iron pledge. This hesitation is very instructive about the prime minister. I do not, as some do, think that David Cameron has a secret agenda to take Britain out of the European Union. He has a not-so-secret agenda to try to stay on the side of public opinion, appease his unruly party and cover his right flank against Nigel Farage's buoyant Ukip. Mr Cameron is the most Eurosceptic Tory ever to inhabit Number 10, but if it ever came to the complete crunch, he has several times already indicated that he would vote for Britain's continued membership of the EU. After much agonising, so I suspect would also William Hague and George Osborne, his two most senior cabinet colleagues.

So on the fundamental question, in or out, here is the line-up of forces. On the side of remaining in the European Union: the Lib Dems, the Labour party, an important number of senior Conservatives, the vast majority of business and the vast majority of trades unions. On the side of leaving: a lot of Tories, a few noisy newspapers, hardly any businesses and hardly any trades unionists. That is why I say the outists are unwise to toast victory before the battle has even been properly joined.